A number of processes for preparing formaldehyde from methanol are known (see, for example, Ullmann""s Encyclopaedia of Industrial Chemistry). The processes carried out industrially are predominantly the oxidation
CH3OH+xc2xdO2xe2x86x92CH2O+H2O
over catalysts comprising iron oxide and molybdenum oxide at from 300xc2x0 C. to 450xc2x0 C. (Formox process) and the oxidative dehydrogenation (silver catalyst process) according to:
CH3OHxe2x86x92CH2O+H2H2+xc2xdO2xe2x86x92H2O
at from 800xc2x0 C. to 720xc2x0 C. In both processes, the formaldehyde is first obtained as an aqueous solution. Particularly when used for the preparation of formaldehyde polymers and oligomers, the formaldehyde obtained in this way has to be subjected to costly dewatering. A further disadvantage is the formation of corrosive formic acid, which has an adverse effect on the polymerization, as by-product
The dehydrogenation of methanol enables these disadvantages to be avoided and enables, in contrast to the abovementioned processes, virtually water-free formaldehyde to be obtained directly: 
In order to achieve an ecologically and economically interesting industrial process for the dehydrogenation of methanol, the following prerequisites have to be met: the strongly endothermic reaction should be carried out at high temperatures so that high conversions are achieved. Competing secondary reactions have to be suppressed in order to achieve sufficient selectivity for formaldehyde (without catalysis, the selectivity for the formation of formaldehyde is less than 10% at conversions above 90%). The residence times have to be short or the cooling of the reaction products has to be rapid in order to minimize the decomposition of the formaldehyde which is not thermodynamically stable under the reaction conditions
CH2Oxe2x86x92CO+H2.
Various methods of carrying out this reaction have been proposed; thus, for example. DE-A-37 19 055 describes a process for preparing formaldehyde from methanol by dehydrogenation in the presence of a catalyst at elevated temperature. The reaction is carried out in the presence of a catalyst comprising at least one sodium compound at a temperature of from 300xc2x0 C. to 800xc2x0 C.
J. Sauer and G. Emig (Chem. Eng. Technol. 1995, 18, 284-291) were able to set free a catalytically active species, which they presumed to be sodium, from a catalyst comprising NaAlO2 and LIAlO2 by means of a reducing gas mixture (87% N2+13% H2). This species was able to catalyze the dehydrogenation of methanol introduced at a downstream point in the same reactor, i.e. not coming into contact with the catalyst bed, to give formaldehyde. When using non-reducing gases, only a low catalytic activity was observed.
According to J. Sauer and G. Emig and also results from more recent studies (see, for example, M. Bender et al., paper presented to the 30th annual meeting of German catalyst technologists, Mar. 21-23, 1997), sodium atoms and NaO molecules were identified as species emitted into the gas phase and their catalytic activity for the dehydrogenation of methanol in the gas phase was described. In the known processes, the starting material methanol is always diluted with nitrogen and/or nitrogen/hydrogen mixtures for the reaction.
Although good results are achieved with the known processes, there is nevertheless considerable room for improvement from a technical and economic point of view, particularly because the catalysts employed become exhausted or inactivated over time and the formaldehyde yields are still capable of improvement.
It has surprisingly been found that the yield in the dehydrogenation can be increased if a carrier gas stream which has been brought to a temperature above the actual reaction temperature by heating is introduced into the reactor. By means of such a superheated carrier gas stream, at least part of the heat required for the endothermic dehydrogenation reactor can be introduced.
An advantage here is that the heat of reaction does not have to be transferred to the gas stream via a hot wall, i.e. one having a temperature above the reaction temperature, in the reaction zone, but can be introduced directly and more gently for the reaction gases by means of the separate heating and intensive mixing of the various substreams. Decomposition of the unstable formaldehyde and secondary reactions at the high temperatures in the reactor, in particular in the zones close to the wall, can thus be reduced.
The invention accordingly provides a process for preparing formaldehyde from methanol by dehydrogenation in a reactor in the presence of a catalyst at a temperature in the range from 300 to 1000xc2x0 C., wherein a carrier gas stream which has a temperature above the dehydrogenation temperature is fed to the reactor.
The temperature difference between carrier gas stream and dehydrogenation temperature is preferably at least 20xc2x0 C., particularly preferably from 40 to 250xc2x0 C.
The superheated gas stream can be fed directly into the reaction zone or all or part of it can be brought into contact with a primary catalyst (see below) beforehand.
The preferred temperatures for the superheated gas stream are from 600 to 1000xc2x0 C., particularly preferably from 700 to 900xc2x0 C. Preferred temperatures for the dehydrogenation of the methanol are from 500 to 900xc2x0 C.; particular preference is given to temperatures of from 600 to 800xc2x0 C.
The carrier gas stream or streams can consist of a reducing or non-reducing gas, for example H2/CO mixtures or nitrogen, preferably the by-products of the dehydrogenation.